Generally, a machine tool spindle has a fixed power input from a motor and a fixed power output, limiting its use to a certain range of machining operations and loads. There are, however, applications in which a multiple input or a multiple output is desirable, and may be achieved by a differential or similar type of drive system. Differential drive systems are used in automotive applications to drive, from one input-torque shaft, multiple output shafts connected to wheels, such as in the case of four-wheel drive vehicles. Conversely, multiple input shafts may drive a single propeller shaft, such as in fixed wing aircraft, as described in U.S. Pat. No. 4,829,850. Another related application is the use of hybrid transmissions for vehicles, using two- or four-mode input-split electromechanical transmissions, as described in U.S. Pat. Nos. 5,551,078, 5,577,973, and 5,931,757 to Schmidt, the contents of which are incorporated herein by reference.
Many of the conventional multiple input or multiple output differential drive systems require a combination of 2.sup.n differentials to provide equal power contribution among the several inputs or outputs, respectively. Greater freedom may be achieved by combining multiple epicyclic gear mechanisms in series, or by combining epicyclic and bevel gear systems, as described in U.S. Pat. Nos. 5,423,726 and 5,435,790, which relate to differential drives with plural or N outputs, respectively. U.S. Pat. Nos. 5,423,726 and 5,435,790 are incorporated herein by reference.
The gear drive mechanisms of the prior art, including those mentioned, are not well-suited to situations in which it is required that the same power spindle performs different machining operations with different power requirements. Nor are existing systems efficient in terms of power use, conservation of resources and reduction of machine set-up time, or space-conservation.
In addition to these considerations, it would also be desirable for the spindle itself to have exactly the power required for any specific operation in a given range of operations, and to be able to modify quickly the power range. The traditional epicyclic gear train consists of a sun gear rotatable about a center axis, one set of planetary gears, an arm or carrier rotatable about the center axis and supporting the planetary gears, and a ring gear rotatable about the center axis. The planetary gears are engaged with the ring gear and the sun gear. This epicyclic gear train may be used for a two-input, one-output application, but the contribution of angular velocity or power between the two inputs cannot be equal. An equal contribution of power is traditionally achieved by employing an additional set of bevel gears at the price of significantly increasing the space requirements, as shown in FIG. 1.